The invention concerns a device using high-gradient magnetic separation techniques to extract magnetizable particles from a flowing fluid medium. The device includes a filter structure that contains a number of wire nets made of non-corroding, ferromagnetic material with a predetermined mesh width and wire gauge which are arranged at least approximately perpendicularly to the direction of flow of the medium and, viewed along the direction of flow, relatively close behind one another. The wire nets are placed in a magnetic field that is directed substantially parallel or antiparallel to the direction of flow of the medium.
A separation device of the type just described is disclosed in the German Pat. No. 26 28 095. In the magnetic separation process, advantage is taken of the fact that, in a suitably arranged magnetic field, a magnetizable particle is subject to a force that moves it or holds it in place against the other forces acting on it. The latter forces include, for example, the force of gravity or hydrodynamic forces of friction in a liquid medium. Separation processes of this kind are intended, for example, for steam or cooling-water circuits in both conventional and nuclear power plants. Particles, which have usually been produced by corrosion, are suspended in the liquid or gaseous medium in these circuits. It is difficult to remove these particles from the medium with the aid of a magnetic separation process, however, because they differ greatly in their chemical composition, their particle size and their magnetizability. For example, the corrosion products in the secondary circuit of a nuclear power plant consist of various iron oxides, of which the largest part by weight is ferrimagnetic magnetite (Fe.sub.3 O.sub.4), the second largest is antiferromagnetic hematite (.alpha.-Fe.sub.2 O.sub.3) and the remainder consist of paramagnetic hydroxides.
Mechanical separation devices that retain particles as a result of the small pore width of their filter matrices are indeed not affected in terms of efficiency by the chemical composition and the magnetic properties of the particles; however, there are two main difficulties with these devices: In the first place, the loaded filter matrices can be cleaned only with relative difficulty, so that in most cases they can be used only as fairly expensive disposable filters. In the second place, when the flow-through rate is high, these filter matrices take up a great deal of space, because the filtering surface must be correspondingly enlarged.
With the so-called "ball-filters" that are known in the art (cf., e.g., German Pat. No. 1,277,488) it is possible as a general rule to extract only easily magnetizable particles, in other words, primarily ferromagnetic ones. A device of this kind contains a cylindrical filter holder which is filled with balls made of magnetically soft iron. These are exposed to a magnetic field generated by an electric coil surrounding the filter holder. By means of this magnetic field in connection with the balls, sufficiently high field-strength gradients are obtained to accumulate, at the magnetic poles of the balls, the ferromagnetic particles that are being carried along in the liquid flowing through the filter. The balls are subsequently demagnetized in order to clean the filter. As far as the particles with lower magnetizability is concerned, however, the rate of separation achieved by this familiar device--is other words, the ratio of the concentration in suspended material removed by the ball filter to the corresponding concentration before entry into the filter--is relatively small. The smallest ferromagnetic particles, as well as weakly magnetic (i.e., antiferromagnetic or paramagnetic) particles can be effectively filtered out from a flowing medium by magnetic means only by using separation devices based on the so-called "high-gradient magnetic separation" technique ("HGM" technology) (cf., e.g., "Journal of Magnetism and Magnetic Materials" Vol. 13, 1979, pages 1 to 10). An HGM separation device of this kind is described in the German Pat. No. 26 28 095, referred to above. It contains, in a central filtering space, a filter structure consisting of a number of wire nets, which, viewed along the direction of flow, are placed in a stack relatively close behind one another and arranged perpendicularly to the direction of flow of the medium in a relatively strong magnetic field. This magnetic field is directed in parallel or antiparallel to the direction of flow of the medium in the area of the filter structure and generates there, for example, a magentic induction on the order of 1 Tesla. The wires which form the nets are made of ferromagnetic material and are of very small gauge--for example, less than 0.1 mm. The magnetic field gradients generated in these nets are consequently very high, so that even weakly magnetizable particles can be filtered out. Experience has shown, however, that in circuits with particles suspended in a medium that are of widely differing particle size and magnetizability, the wire nets of this type of separation device become loaded relatively quickly on the inflow side, while only comparatively small quantities are extracted at the nets that are farther along the line of flow. The rate of separation and the maintenance interval--in other words, the period of time between two required cleaning processes--are correspondingly limited for this separation device.